Twig is a flexible, fast, and secure template engine for PHP. It's used to separate the presentation logic from the application logic in web development. Twig files contain HTML markup interspersed with Twig syntax, which allows developers to dynamically generate HTML content based on data passed from the PHP application. This separation of concerns makes code more maintainable, readable, and easier to test. Twig offers features like template inheritance, automatic escaping to prevent security vulnerabilities like XSS, and a powerful expression language for manipulating data within templates. It supports filters, functions, and tests to further enhance the templating process. Twig templates are compiled into optimized PHP code, resulting in excellent performance. The syntax is designed to be concise and easy to learn, making it a popular choice for PHP developers of all skill levels. It is commonly used in frameworks like Symfony and Drupal, but can also be used in any PHP project.